The kinematics of 3:1-merger remnants and the formation of low-luminosity elliptical galaxies

Abstract

We test the formation of low-luminosity elliptical galaxies through collisionless mergers of unequal-mass disk galaxies. The kinematic properties of a small survey of simulated merger end-products with initial disk mass ratios of 3:1 is compared to a sample of seven low-luminosity galaxies observed by Rix et al. that were chosen photometrically to be "ellipticals". In this paper, we go beyond a comparison in terms of global properties (using one number to characterize a model or a galaxy e.g. <a4>, ellipticity at some fixed radius or central velocity dispersion) and examine the detailed kinematics as function of galactocentric distance. The merger remnants are ``observed'' through a slit along the major and minor axis, using a pixel binning and slit width similar to the one used during the spectroscopic observations. We compare the rotational support (v/sigma) of the merger remnants, the normalized rotation on the minor axis and the major axis Gauss-Hermite moments h3, to that of the observed galaxies. The N-body remnants are very flattened when viewed edge-on (<epsilon> = 0.6) and should be inclined before making a fair comparison with the Rix et al. data set (which has <epsilon> = 0.3). When the merger remnants are appropriately inclined, their v/sigma profiles rise slower than the observed ones. Note that even when the remnants are viewed edge-on, the v/sigma profiles do not match the observations. The detailed comparison of the observations with purely collisionless 3:1-merger remnants suggests that this kind of dissipationless merger is not likely to be the dominant formation channel for low-luminosity elliptical galaxies. (abridged)